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"You can look healthy, but have a lot of fat internally, which can have a detrimental effect on your health."

Even apparently slim people can have internal fat collected inside or around the liver, gut, heart and pancreas, or streaked through under-used muscles, he said.

Prof Bell and his team began using the MRI scanner to seek out internal fat while researching type 2 diabetes, the version of the disease that develops later in life and which is normally associated with obesity. His suspicions arose when several people in the study who were slender had the medical markers for type 2 diabetes.

The findings are raising questions about the BMI (Body Mass Index), the indicator of obesity used by doctors and public health campaigners. The BMI is a relatively crude measure which takes into account a person's height and weight. To calculate a BMI, a person's weight in kilograms is divided by their height in metres squared.

Some doctors believe the BMI is flawed because it pays no attention to the nature of the weight.

A rugby player, for instance, with heavier than usual muscles will come out with a high BMI and could be classified as overweight, even though they have low levels of internal fat.

"When you exercise you tend to burn the internal fat," said Prof Bell. "Lifestyle changes have to include physical activity. It is not just enough to diet."

Imperial College researchers have discovered that a typical woman has around three litres of internal, or visceral fat, while a man has around five litres. However, overall, women have 37 litres of fat, compared with 35 litres in men.

Ben Schwartz, 28, who was scanned at Hammersmith Hospital last week, discovered his internal fat levels are too high even though he is outwardly healthy.

The MRI showed that he had 3.75 litres of internal fat out of a total of 20.75 litres in his body.

Mr Schwartz, who works for a television company, said he spent half his day at a desk, and half the day outside, but did not have the time to go to the gym.